This invention relates to radio broadcasting and more particularly to a system for identifying an FM stereophonic broadcast which has been specially encoded.
In recent years an increasing number of FM stereophonic broadcast stations in the United States and other countries have incorporated an encoding technique in their transmissions known commercially as Dolby FM encoding, which includes the use of Dolby B-type compression, a type of high-frequency compression, and the use of 25 microsecond pre-emphasis in place of the conventional 75 microsecond pre-emphasis. Details of Dolby FM encoding are set forth in the articles "Optimum Use of Noise Reduction in FM Broadcasting," by Ray M. Dolby, Journal of the Audio Engineering Society, June 1973, vol. 21, No. 5, pp. 357-362 and "Dolby B-Type Noise Reduction for FM Broadcasts," by D. P. Robinson, Journal of the Audio Engineering Society, June 1973, vol. 21, No. 5, pp. 351-356. At the same time, growing numbers of FM stereophonic receiving devices are being equipped with the complementary Dolby decoder. A problem encountered by users of such receiving equipment is that there is no positive way to determine if a particular station is actually transmitting the specially encoded signal. Usually, the user relies on a listing of stations known to be broadcasting the specially encoded signals and accordingly switches in the decoder based on a belief that the listing is accurate and up to date.
Therefore, a system for identifying specially encoded broadcasts so that the user can readily recognize such broadcasts and switch in the decoder or so that the receiving device can automatically switch in the decoder while indicating reception of such a broadcast, by a lamp for example, is highly desirable.
Such a system should preferably fall within the parameters of a standard FM stereophonic broadcast in order to simplify implementation. The audio signal must not be affected in any way, such as by notching of the spectrum. The system must also have a low circuit cost while maintaining high operating reliability. The system should preferably accommodate more than one separately identifiable mode so that other encoded functions (e.g. quadraphonic) may be controlled.